Many oilfield operators are faced with challenges in flow assurance due to formation of deposits and soaps in the oilfield, generally termed naphthenates. Naphthenate deposits in crude oil production cause major problems for petroleum producers by obstructing pipelines and screens and causing production irregularities that result in unplanned and expensive production shutdowns. The problem of naphthenate deposits has recently become more pronounced as more oilfields that contain immature, heavy oils with higher naphthenic acid content begin production. Examples of oilfields associated with the formation of naphthenate deposits include those found offshore in Great Britain, West Africa, the North West Shelf in Australia, Bohai Bay in China, Kutei Basin in Indonesia and the Gulf of Mexico.
Naphthenate deposits form mainly during upstream operations, commonly at the oil/water cutoff point. Calcium and sodium naphthenate deposits are solid and emulsion-like deposits formed by the interaction of naphthenic acids and divalent (Ca2+, Fe2+ and Mg2+) or monovalent (Na+, K+) ions present in produced waters. Calcium naphthenates occur as a viscous, sticky solid or sludge commonly found in oil-water separators and electrostatic treaters. Interestingly, calcium naphthenates harden when they are exposed to air and cooling. Crude oils with medium to heavy American Petroleum Institute (API) gravities (API values of 18-27° API) rich in naphthenic acids, with a high total acid number (TAN=0.80-8.0 mg KOH/(g oil)) are at risk to react with divalent cations to form calcium naphthenates, such as calcium naphthenate. In contrast to calcium naphthenates, naphthenates formed with monovalent cations occur as a very stable emulsion, soap, or sludge, and are associated with light API gravities (typically 32 to 41° API) crude oils with a low TAN (0.05-0.60) and high concentration of volatile fatty acids.
Naphthenic acids are saturated acyclic or cyclic carboxylic acids in which the carboxylic acid group is attached to the aliphatic side chain or a cycloaliphatic ring (single ring or multiple fused rings). The characterization of naphthenic acids has always been a challenge, especially for naphthenic acids associated with formation of naphthenate deposits. However, naphthenic acids found in Norwegian calcium naphthenate deposits are currently known by the generic name “ARN acids.” ARN acids are tetraprotic naphthenic acids with molecular weights generally ranging from 1227 to 1235 Da. They are believed to include a head to head linked isoprenoid 20-bis-16, 16′ biphytane carbon skeleton, with 4-6 cyclopentane rings. The homologous series of ARN acids corresponds to empirical formulas, C80H138O8, C80H138O8, C80H140O8, C80H142O8, C80H144O8, and C80H146O8 with double bound equivalent (DBE) values ranging from 8 to 12. As used herein, DBE refers to double bond equivalents and is equal to the number of rings plus the number of carbon double bonds, e.g., C═C, C═O, etc.
As ARN acids are tetraprotic carboxylic acids, a DBE range from 8 to 12 indicates 4 to 8 rings in the hydrocarbon structure. However, the presence of ARN acids does not guarantee the formation of naphthenate deposits. Currently, there is no means of predicting whether a crude oil composition will form naphthenate deposits until the naphthenate deposits are found in the crude oil operation. Once naphthenate deposits are discovered, shutdowns are required to remove the deposits and it would be expensive to retrofit the crude oil operation to reduce or eliminate the source of the deposits.